Fuel cell system and vehicle equipped with the same

Cross-reference to related application

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2011-135905 filed on Jun. 20, 2011, of which the contents are incorporated herein by reference.

Background of the invention

1. Field of the invention

The present invention relates to a fuel cell system including a voltage regulator and an energy storage device which are connected in series to each other, a fuel cell connected in parallel to the serially connected voltage regulator and energy storage device, and a load driven by a combined power source of the energy storage device and the fuel cell. Further, the present invention relates to a vehicle equipped with such a fuel cell system. Such a fuel cell system and such a vehicle equipped with the fuel cell system make it possible to suppress degradation of the fuel cell, and improve the system efficiency.

2. Description of the related art

A fuel cell employs a membrane electrode assembly (MEA) which includes an anode, a cathode, and a solid polymer electrolyte membrane interposed between the anode and the cathode. The solid polymer electrolyte membrane is formed by impregnating a thin membrane of perfluorosulfonic acid with water, for example. Each of the cathode and the anode has a gas diffusion layer such as a carbon paper, and an electrode catalyst layer of catalyst particles of platinum alloy or the like (hereinafter also referred to as the Pt catalyst) supported on porous carbon particles. The carbon particles are deposited uniformly on the surface of the gas diffusion layer. The electrode catalyst layer of the anode and the electrode catalyst layer of the cathode are fixed to both surfaces of the solid polymer electrolyte membrane, respectively.

A technique of suppressing degradation of the fuel cell is proposed in JP 2007-005038 A. In the technique proposed in JP 2007-005038 A, power generation of the fuel cell is performed such that an oxidation reduction electric potential where sintering phenomenon of the Pt catalyst (aggregation of the Pt catalyst) occurs is avoided.

Summary of the invention

In the technique according to JP 2007-005038 A, in order to continuously avoid the oxidation reduction electric potential, it is required to increase or decrease electric power outputted from the fuel cell relative to electric power required by load such as a traction motor driven by the electric power generated in the fuel cell. In this case, with excessive electric power generated in the fuel cell, a battery (energy storage device) is charged by stepping down the voltage through chopping operation of a DC/DC converter. If electric power generated by the fuel cell is not sufficient for electric power required by the load, the voltage of the battery is stepped up by chopping operation of the DC/DC converter to provide assistance by the battery (to compensate for the shortage of electric power by the fuel cell).

However, in the technique according to JP 2007-005038 A, in order to continuously avoid the oxidation reduction electric potential, chopping operation of the DC/DC converter occurs frequently, and the switching loss of the DC/DC converter is large. Therefore, the system efficiency of the fuel cell system is low disadvantageously.

The present invention has been made taking the problem of this type into account, and an object of the present invention is to provide a fuel cell system and a vehicle equipped with the fuel cell system which make it possible to prevent degradation of a fuel cell, and improve the system efficiency of the fuel cell system.

A fuel cell system according to the present invention includes a fuel cell having catalyst, for performing power generation by inducing reaction of oxygen or hydrogen at the catalyst, a gas supply unit for supplying at least one of the oxygen and the hydrogen to the fuel cell, an energy storage device in which an output voltage thereof changes depending on an amount of electric power stored in the energy storage device, a voltage regulator provided in parallel to the fuel cell, and in series with the energy storage device for regulating an output voltage of the fuel cell, a load driven by electric power outputted from the fuel cell, and a control unit for detecting electric power required by the load and controlling the fuel cell, the gas supply unit, and the voltage regulator. In a case where the output voltage of the energy storage device is outside a voltage range of the fuel cell where oxidation-reduction proceeds, the control unit controls the voltage regulator to be placed in a direct connection state, and controls the gas supply unit so as to regulate concentration of the oxygen or the hydrogen supplied to the fuel cell in accordance with a target power generation electric power determined based on the electric power required by the load.

In the present invention, in the case where the output voltage of the energy storage device is outside a voltage range of the fuel cell where oxidation-reduction proceeds, the control unit controls the voltage regulator to be placed in the direct connection state (output voltage of the energy storage device.apprxeq.output voltage of the fuel cell), not in a voltage step-up/step-down state where a voltage step-up/step-down operation is performed, and controls the gas supply unit so as to regulate concentration of the oxygen or the hydrogen supplied to the fuel cell in accordance with a target electric power for the power generation determined based on the electric power required by the load. Therefore, degradation of the fuel cell is prevented, and power loss related to the voltage step-up/step-down operation of the voltage regulator is reduced.

The above case where the output voltage of the energy storage device is outside the voltage range of the fuel cell where oxidation-reduction proceeds means any of the following cases

and (2):

The output voltage is within a voltage range where reduction of the catalyst proceeds stably, below the voltage range of the fuel cell where oxidation-reduction proceeds; or

The output voltage is within a voltage range where oxidation of the catalyst proceeds stably, above the voltage range of the fuel cell where oxidation-reduction proceeds.

Further, preferably, the fuel cell system includes an outside temperature sensor for measuring an outside temperature, and if the outside temperature measured by the outside temperature sensor is a threshold temperature or more, the control unit controls the voltage regulator to be placed in the direct connection state, i.e., the energy storage device and the fuel cell are controlled to be placed in the direct connection state, at a voltage within the voltage range where reduction of the catalyst proceeds stably. If the outside temperature is lower than the threshold temperature, energy may be required, e.g., for warming up the fuel cell system at the time of restarting operation of the fuel cell system. In this case, the voltage of the energy storage device is kept high without implementing the direct connection control. In this manner, sufficient electric power can be secured.

Moreover, when the electric power required by the load is high, the control unit regulates the output voltage of the fuel cell by the voltage regulator to supply electric power from the fuel cell and the energy storage device to the load, and when the electric power required by the load is low, the control unit controls the voltage regulator to be placed in the direct connection state to cover the electric power required by the load mainly by the electric power from the fuel cell. For example, assuming that the fuel cell system is mounted in a vehicle, the state where electric power required by the load is high indicates a state where the vehicle equipped with the fuel cell system is in the middle of hill climbing or the like.

Further, at the time of controlling the voltage regulator to be placed in the direct connection state, the control unit controls the target SOC value of the energy storage device to a threshold SOC value or less. The threshold SOC value corresponds to the lower limit voltage of the voltage range of the fuel cell where oxidation-reduction proceeds. In this manner, it is possible to place the energy storage device and the fuel cell in the direct connection state more frequently.

According to the present invention, there is also provided a vehicle equipped with the above-mentioned fuel cell system.

In the present invention, in the case where the output voltage of the energy storage device is outside a voltage range of the fuel cell where oxidation-reduction proceeds, the control unit controls the voltage regulator to be placed in the direct connection state, and controls the gas supply unit so as to regulate concentration of the oxygen or the hydrogen supplied to the fuel cell in accordance with a target power generation electric power determined based on electric power required by the load. Thus, it becomes possible to suppress degradation of the fuel cell, and improve the system efficiency of the fuel cell system.

The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example.

Brief description of the drawings

FIG. 1 is a diagram schematically showing an overall structure of a fuel cell vehicle equipped with a fuel cell system according to an embodiment of the present invention;

FIG. 2 is a block diagram showing a power system of the fuel cell vehicle;

FIG. 3 is a diagram schematically showing a structure of a fuel cell unit according to the embodiment;

FIG. 4 is a circuit diagram showing details of a DC/DC converter according to the embodiment;

FIG. 5 is a flow chart showing basic control (main routine) in an electronic control unit (ECU);

FIG. 6 is a flow chart of calculating a system load;

FIG. 7 is a graph showing the relationship between the current rotation number of a motor and the estimated electric power consumed by the motor;

FIG. 8 is a graph showing an example of the relationship between the voltage of a fuel cell of a fuel cell stack and degradation of the fuel cell;

FIG. 9 is a cyclic voltammetry diagram showing an example of the progress of oxidation and the progress of reduction in the cases of different varying speeds in the voltage of the fuel cell;

FIG. 10 is a graph showing a normal current-voltage characteristic of a fuel cell;

FIG. 11 is a graph showing the relationship between the cathode stoichiometric ratio and the cell current;

FIG. 12 is a flow chart illustrating a basic control mode according to power generation control of a fuel cell;

FIG. 13 is a graph showing a plurality of power supply modes (e.g., basic control mode) in the fuel cell;

FIG. 14 is a graph showing the relationship between the SOC value of a battery and the charging/discharging coefficient;

FIG. 15 is a graph showing the relationship between the target FC current and the target oxygen concentration;

FIG. 16 is a graph showing the relationship between the target FC current, and the target air pump rotation number and the target water pump rotation number;

FIG. 17 is a graph showing the relationship between the target FC current, and the target opening degree of a back pressure valve;

FIG. 18 is a flow chart showing torque control of the motor;

FIG. 19 is a graph showing the relationship between the power generation electric power and the power generation efficiency of the fuel cell;

FIG. 20 is a flow chart illustrating operation of a first embodiment;

FIG. 21 is a time chart comparing, and illustrating the first embodiment and a technique according to basic control;

FIG. 22 is a diagram schematically showing a fuel cell unit according to a second embodiment;

FIG. 23 is a graph showing the relationship between the valve opening degree of a circulation valve and the oxygen concentration in a cathode channel;

FIG. 24 is a flow chart illustrating operation of the second embodiment;

FIG. 25 is a graph showing the relationship between the target SOC value and the battery voltage in association with the fuel cell voltage;

FIG. 26 is a time chart comparing, and illustrating a conventional technique and a third embodiment;

FIG. 27 is a block diagram schematically showing a structure of a first modified example of the fuel cell system;

FIG. 28 is a block diagram schematically showing a structure of a second modified example of the fuel cell system; and

FIG. 29 is a block diagram schematically showing a structure of a third modified example of the fuel cell system.

Description of the preferred embodiments

FIG. 1 is a diagram schematically showing the overall structure of a fuel cell vehicle 10 (hereinafter referred to as the "FC vehicle 10") equipped with a fuel cell system 12 (hereinafter referred to as the "FC system 12") according to an embodiment of the present invention. FIG. 2 is a block diagram showing a power system of the FC vehicle 10. As shown in FIGS. 1 and 2, the FC vehicle 10 includes a traction motor 14 (drive motor) and an inverter (bidirectional DC-AC converter) 16 in addition to the FC system 12.

The FC system 12 includes a fuel cell unit 18 (hereinafter referred to as the "FC unit 18"), a high voltage battery (hereinafter referred to as the "battery 20") (energy storage device), a DC/DC converter (voltage regulator) 22, and an electronic control unit 24 (hereinafter referred to as the "ECU 24").

The motor 14 generates a driving force based on the electric power supplied from the FC unit 18 and the battery 20, and rotates wheels 28 using the driving force through a transmission 26. Further, the motor 14 outputs electric power generated by regeneration (regenerative electric power Preg) [W] to the battery 20 or the like (see FIG. 2).

The inverter 16 (also referred to as PDU (power drive unit)) has three phase full bridge structure, and carries out DC/AC conversion to convert direct current into alternating current in three phases. The inverter 16 supplies the alternating current to the motor 14, and supplies the direct current after AC/DC conversion as a result of regeneration of the motor 14 to the battery 20 or the like through a DC/DC converter 22.

It should be noted that the motor 14 and the inverter 16 are collectively referred to as a load 30 (also referred to as a main load 30 in a case where it is necessary to distinguish between the load 30 and loads of auxiliary devices (auxiliary device loads) 31 to be described later). The main load 30 and the auxiliary device loads 31 will be collectively referred to as a load 33 (also referred to as a total load 33).

FIG. 3 is a diagram schematically showing a structure of the FC unit 18. The FC unit 18 includes a fuel cell stack 40 (hereinafter referred to as the "FC stack 40" or the "FC 40"), an anode system 54 for supplying hydrogen (fuel gas) to, and discharging the hydrogen (fuel gas) from anodes of the FC stack 40, a cathode system 56 for supplying the air (oxygen-containing gas) to, and discharging the air (oxygen-containing gas) from cathodes of the FC stack 40, a cooling system 58 for circulating coolant water (coolant) to cool the FC stack 40, and a cell voltage monitor 42.

For example, the FC stack 40 is formed by stacking fuel cells (hereinafter referred to as the "FC cells") each including an anode, a cathode, and a solid polymer electrolyte membrane interposed between the anode and the cathode.

The anode system 54 includes a hydrogen tank 44 (gas supply unit), a regulator 46, an ejector 48, and a purge valve 50. The hydrogen tank 44 contains hydrogen as the fuel gas. The hydrogen tank 44 is connected to the inlet of an anode channel 52 of FC 40 through a pipe 44a, a regulator 46, a pipe 46a, an ejector 48, and a pipe 48a. Thus, the hydrogen in the hydrogen tank 44 can be supplied to the anode channel 52 through the pipe 44a or the like. A shut-off valve (not shown) is provided in the pipe 44a. At the time of power generation of the FC stack 40, the shut-off valve is opened by the ECU 24.

The regulator 46 regulates the pressure of the supplied hydrogen to a predetermined value, and discharges the hydrogen. That is, the regulator 46 regulates the pressure on the downstream side (pressure of the hydrogen on the anode side) in response to the pressure (pilot pressure) of the air on the cathode side supplied through a pipe 46b. Therefore, the pressure of the hydrogen on the anode side is linked to the pressure of the air on the cathode side. As described later, by changing the rotation number or the like of an air pump 60 so as to change the oxygen concentration, the pressure of the hydrogen on the anode side changes as well.

The ejector 48 generates a negative pressure by ejecting hydrogen from the hydrogen tank 44 through a nozzle. By this negative pressure, the anode off gas can be sucked from a pipe 48b.

The outlet of the anode channel 52 is connected to a suction port of the ejector 48 through the pipe 48b. The anode off gas discharged from the anode channel 52 flows through the pipe 48b and again into the ejector 48 to allow circulation of the anode off gas (hydrogen).

The anode off gas contains hydrogen that has not been consumed in the electrode reaction at the anodes, and water vapor. Further, a gas-liquid separator (not shown) is provided at the pipe 48b for separating/recovering water components (condensed water (liquid) and water vapor (gas)) in the anode off gas.

Part of the pipe 48b is connected to a dilution device (not shown) provided in a pipe 64c, through a pipe 50a, a purge valve 50, and a pipe 50b. When it is determined that power generation of the FC stack 40 is not performed stably, the purge valve 50 is opened for a predetermined period in accordance with an instruction from the ECU 24. In the dilution device, the hydrogen in the anode off gas from the purge valve 50 is diluted by the cathode off gas and discharged to atmosphere.

The cathode system 56 includes the air pump 60 (gas supply unit), a humidifier 62, and a back pressure valve 64.

The air pump 60 compresses the external air (air), and supplies the compressed air to the cathode. A suction port of the air pump 60 is connected to the outside (outside of the vehicle, outside air of the vehicle) through a pipe 60a, and an ejection port of the air pump 60 is connected to the inlet of a cathode channel 74 through a pipe 60b, the humidifier 62, and a pipe 62a. When the air pump 60 is operated in accordance with an instruction from the ECU 24, the air pump 60 sucks the air outside the vehicle through the pipe 60a, compresses the sucked air, and supplies the compressed air to the cathode channel 74 of FC 40 through the pipe 60b or the like under pressure.

The humidifier 62 has a plurality of hollow fiber membranes 62e having water permeability. The humidifier 62 humidifies the air flowing toward the cathode channel 74 through the hollow fiber membranes 62e by exchanging water components between the air flowing toward the cathode channel 74 and the highly humidified cathode off gas discharged from the cathode channel 74.

A pipe 62b, the humidifier 62, a pipe 64a, the back pressure valve 64, the pipe 64b, and the pipe 64c are provided at the outlet of the cathode channel 74. The cathode off gas (oxygen-containing off gas) discharged from the cathode channel 74 is discharged from the pipe 64c to the outside of the vehicle (to atmosphere) through the pipe 62b or the like.

For example, the back pressure valve 64 is a butterfly valve, and the opening degree of the back pressure valve 64 is controlled by the ECU 24 to regulate the pressure of the air in the cathode channel 74. More specifically, if the opening degree of the back pressure valve 64 becomes small, the pressure of the air in the cathode channel 74 is increased, and oxygen concentration per volume flow rate (volume concentration) becomes high. Conversely, if the opening degree of the back pressure valve 64 becomes large, the pressure of the air in the cathode channel 74 is decreased, and oxygen concentration per volume flow rate (volume concentration) becomes low.

A temperature sensor 72 is attached to the pipe 64a. The temperature sensor 72 detects the temperature of the cathode off gas, and outputs the detected temperature to the ECU 24.

The cooling system 58 includes a water pump 80 and a radiator (heat radiator) 82. The water pump 80 circulates the coolant water (coolant), and an ejection port of the water pump 80 is connected to a suction port of the water pump 80 through a pipe 80a, a coolant channel 84 of the FC stack 40, a pipe 82a, the radiator 82, and a pipe 82b in the order listed. When the water pump 80 is operated in accordance with an instruction from the ECU 24, the coolant water is circulated between the coolant channel 84 and the radiator 82 to cool the FC stack 40.

The cell voltage monitor 42 is a measurement instrument for detecting the cell voltage Vcell of each of unit cells of the FC stack 40. The cell voltage monitor 42 includes a monitor body, and a wire harness connecting the monitor body with each of the unit cells. The monitor body scans all of the unit cells at predetermined intervals to detect the cell voltage Vcell of each cell, and calculates the average cell voltage and the lowest cell voltage. Then, the monitor body outputs the average cell voltage and the lowest cell voltage to the ECU 24.

As shown in FIG. 2, electric power from the FC stack 40 (hereinafter referred to as the "FC electric power Pfc") is supplied to the inverter 16 and the motor 14 (during power running), and to the high voltage battery 20 through the DC/DC converter 22 (during charging). Further, the FC electric power Pfc is supplied to the air pump 60, the water pump 80, the air conditioner 90, a downverter 92 (step down converter), a low voltage battery 94, an accessory 96, and the ECU 24. A backflow prevention diode 98 is disposed between the FC stack 40 and the inverter 16 and the DC/DC converter 22. Further, the power generation voltage of the FC 40 (hereinafter referred to as the "FC voltage Vfc") is detected by a voltage sensor 100 (FIG. 4), and the power generation current of the FC 40 (hereinafter referred to as the FC current Ifc") is detected by a current sensor 102. The FC voltage Vfc and the FC current Ifc are outputted to the ECU 24.

The battery 20 is an energy storage device (energy storage) containing a plurality of battery cells. For example, a lithium-ion secondary battery, a nickel hydrogen secondary battery, or a capacitor can be used as the battery 20. In the embodiment, the lithium-ion secondary battery is used. The output voltage [V] of the battery 20 (hereinafter referred to as the "battery voltage Vbat or primary voltage V1") is detected by a voltage sensor 120, and the output current [A] of the battery 20 (hereinafter referred to as the "battery current Ibat or primary current I1") is detected by a current sensor 124. The battery voltage Vbat and the battery current Ibat are outputted to the ECU 24. The remaining battery level (state of charge) (hereinafter referred to as the "SOC") [%] of the battery 20 is detected by a SOC sensor 104 (FIG. 2), and outputted to the ECU 24.

Under the control of the ECU 24, the DC/DC converter 22 controls targets to which the FC electric power Pfc [W] from the FC unit 18, the electric power [W] supplied from the battery 20 (hereinafter referred to as the "battery electric power Pbat"), and the regenerative electric power from the motor 14 are supplied.

FIG. 4 shows an example of the DC/DC converter 22 in the present embodiment. As shown in FIG. 4, one side of the DC/DC converter 22 is connected to the primary side 1S where the battery 20 is provided, and the other side of the DC/DC converter 22 is connected to the secondary side 2S, which is connection points between the load 33 and the FC stack 40.

The DC/DC converter 22 is basically a chopper type step up/down voltage converter for increasing the voltage on the primary side 1S (primary voltage V1=Vbat) [V] to the voltage on the secondary side 2S (secondary voltage V2) [V](V1.ltoreq.V2), and decreasing the secondary voltage V2 to the primary voltage V1 (V1=Vbat).

As shown in FIG. 4, the DC/DC converter 22 includes a phase arm UA interposed between the primary side 1S and the secondary side 2S, and a reactor 110.

The phase arm UA includes an upper arm element (an upper arm switching element 112 and a diode 114) as a high-side arm and a lower arm element (a lower arm switching element 116 and a diode 118) as a low-side arm. For example, MOSFET, IGBT, or the like is adopted in each of the upper arm switching element 112 and the lower arm switching element 116.

The reactor 110 is interposed between the middle point (common connection point) of the phase arm UA and the positive electrode of the battery 20. The reactor 110 is operated to release and accumulate energy during voltage conversion between the primary voltage V1 and the secondary voltage V2 by the DC/DC converter 22.

The upper arm switching element 112 is turned on when high level of a gate drive signal (drive voltage) UH is outputted from the ECU 24, and the lower arm switching element 116 is turned on when high level of a gate drive signal (drive voltage) UL is outputted from the ECU 24.

The ECU 24 detects primary voltage V1 [V] by a voltage sensor 120 provided in parallel with a smoothing capacitor 122 on the primary side 1S, and detects electrical current on the primary side 1S (primary current I1) [A] by a current sensor 124. Further, the ECU 24 detects secondary voltage V2 [V] by a voltage sensor 126 provided in parallel with the smoothing capacitor 128 on the secondary side 2S, and detects electrical current on the secondary side 2S (secondary current I2) [A] by a current sensor 130.

At the time of stepping up the voltage of the DC/DC converter 22, at the first timing, the gate drive signal UL is switched to high level, and the gate drive signal UH is switched to low level. Electricity from the battery 20 is stored in the reactor 110 (through a current path extending from the positive side of the battery 20 through the reactor 110 and the lower arm switching element 116 to the negative side of the battery 20). At the second timing, the gate drive signal UL is switched to low level, and the gate drive signal UH is switched to low level. Electricity stored in the reactor 110 is supplied to the secondary side 2S through the diode 114 (through a current path extending from the positive side of the battery 20 through the reactor 110, the diode 114, the positive side of the secondary side 2S, the load 33 or the like, and the negative side of the secondary side 2S to the negative side of the battery 20). Thereafter, the first timing and the second timing are repeated during the period in which the voltage is stepped up.

At the time of stepping down the voltage of the DC/DC converter 22, at the first timing, the gate drive signal UH is switched to high level, and the gate drive signal UL is switched to low level. Electricity from the secondary side 2S (FC stack 40 or the load 33 during electric power regeneration by the motor 14) is stored in the reactor 110, and with the electricity, the battery 20 is charged. At the second timing, the gate drive signal UH is switched to low level, and the gate drive signal UL is switched to low level. Electricity stored in the reactor 110 is supplied to the battery 20 through the diode 118, and with the electricity, the battery 20 is charged. As can be seen from FIG. 2, the regenerative electric power from the motor 14 can be supplied to loads 31 of auxiliary devices such as the air pump 60. Thereafter, the first timing and the second timing are repeated during the period in which the voltage is stepped down.

As described above, the DC/DC converter 22 is operated as a chopper type converter. Further, the DC/DC converter 22 can be operated as a direct connection type converter. In the case where the DC/DC converter 22 is operated as the direct connection type converter, when the gate drive signal UH is switched to high level at the duty of 100 [%], and the gate drive signal UL is switched to low level at the duty of 0 [%], thereby discharging electric power from the battery 20, electrical current is supplied from the primary side 1S to the secondary side 2S through the diode 114 (e.g., electric power is supplied from the battery 20 to the load 33). At the time of charging the battery 20 with electric power, the electric power is supplied from the secondary side 2S to the battery 20 through the upper arm switching element 112 (e.g., regenerative electric power is supplied from the motor 14 to the battery 20).

The ECU 24 controls the motor 14, the inverter 16, the FC unit 18, the auxiliary device loads 31, the battery 20, the DC/DC converter 22, etc. through a communication line 140 (see e.g., FIG. 1). For implementing the control, programs stored in a memory (ROM) are executed, and detection values obtained by various sensors such as the cell voltage monitor 42, the flow rate sensors 68, the temperature sensor 72, the voltage sensors 100, 120, 126, the current sensors 102, 124, 130, and the SOC sensor 104 are used.

In addition to the above sensors, the various sensors herein include an opening degree sensor 150, a motor rotation number sensor 152, a vehicle velocity sensor 154, and an outside temperature sensor 159 (FIG. 1). The opening degree sensor 150 detects the opening degree (accelerator opening degree) .theta.p [degrees] of an accelerator pedal 156, which is an accelerator pedal angle, and the rotation number sensor 152 detects the rotation number Nm [rpm] of the motor 14. The vehicle velocity sensor 154 detects the vehicle velocity Vs [km/h] of the FC vehicle 10. The outside temperature sensor 159 detects an outside temperature Ta [.degree. C.]. Further, a main switch 158 (hereinafter referred to as the "main SW 158") is connected to the ECU 24. The main SW 158 switches between supply and non-supply of the electric power from the FC unit 18 and the battery 20 to the motor 14. This main SW 158 can be operated by a user and corresponds to the ignition switch of an engine vehicle.

The control means including ECU 24 includes a microcomputer. Further, as necessary, the control means including the ECU 24 has a timer and input/output (I/O) interfaces such as an A/D converter and a D/A converter. The ECU 24 may comprise only a single ECU. Alternatively, the control means including the ECU 24 may comprise a plurality of ECUs for each of the motor 14, the FC unit 18, the battery 20, and the DC/DC converter 22.

After the load required by the FC system 12, i.e., required by the FC vehicle 10 as a whole is determined based on the state of the FC stack 40, the state of the battery 20, and the state of the motor 14, and also based on inputs (load requests) from various switches and various sensors, the ECU 24 determines allocation (shares) of loads through adjustment, and more specifically determines a good balance among a load which should be assigned to the FC stack 40, a load which should be assigned to the battery 20, and a load which should be assigned to the regenerative power supply (motor 14), and sends instructions to the motor 14, the inverter 16, the FC unit 18, the battery 20, and the DC/DC converter 22.

[Explanation of Basic Control Operation]

Next, operation of basic control in the ECU 24 will be described. On the premise of the basic control, first through third embodiments will be described later.

FIG. 5 is a flow chart showing basic control (main routine) in the ECU 24. In step S1, the ECU 24 determines whether or not the main SW 158 is in an ON state. If the main SW 158 is not in the ON state (S1: NO), step S1 is repeated. If the main SW 158 is in the ON state (S1: YES), the control proceeds to step S2. In step S2, the ECU 24 calculates the load (system load Psys or system requirement load Psys) [W] required by the FC system 12.

In step S3, the ECU 24 performs energy management of the FC system 12 based on the calculated system load Psys. The energy management herein is intended to suppress degradation of the FC stack 40, and improve the efficiency in the output (system efficiency) of the entire FC system 12.

In step S4, based on the results of energy management operation, the ECU 24 implements control for peripheral devices of the FC stack 40, i.e., the air pump 60, the back pressure valve 64, and the water pump 80 (FC power generation control). In step S5, the ECU 24 implements torque control of the motor 14.

In step S6, the ECU 24 determines whether or not the main SW 158 is in an OFF state. If the main SW 158 is not in the OFF state (S6: NO), the control returns to step S2. If the main SW 158 is in the OFF state (S6: YES), the current process is finished.

FIG. 6 is a flow chart for calculating the system load Psys in step S2. In step S11, the ECU 24 reads the opening degree .theta.p of the accelerator pedal 156 from the opening degree sensor 150. In step S12, the ECU 24 reads the rotation number Nm [rpm] of the motor 14 from the rotation number sensor 152.

In step S13, the ECU 24 calculates the estimated electric power Pm [W] consumed by the motor 14 based on the opening degree .theta.p and the rotation number Nm. Specifically, in a map (characteristics) as shown in FIG. 7, the relationship between the rotation number Nm [rpm] of the motor 14 and the estimated electric power Pm [W] consumed by the motor 14 is stored for each opening degree .theta.p. For example, in the case where the opening degree .theta.p is .theta.p1, a characteristic 180 is used. Likewise, in the cases where the opening degrees .theta.p are .theta.p2, .theta.p3, .theta.p4, .theta.p5, and .theta.p6, characteristics 182, 184, 186, 188, and 190 are used, respectively. After the characteristic indicating the relationship between the rotation number Nm and the estimated consumed electric power Pm is determined based on the opening degree .theta.p, the estimated consumed electric power Pm in correspondence with the rotation number Nm is determined based on the determined characteristic. During acceleration in the power running mode, the estimated consumed electric power Pm has a positive value. During deceleration in the regenerating mode, the estimated consumed electric power Pm has a negative value. That is, in this mode, the estimated consumed electric power Pm indicates estimated regenerative electric power.

In step S14, the ECU 24 reads data of the current operating conditions from the load 31 of auxiliary devices. For example, as shown in FIG. 2, the auxiliary devices herein include high voltage auxiliary devices, such as the air pump 60, the water pump 80, and the air conditioner 90, and low voltage auxiliary devices, such as the low voltage battery 94, the accessory 96, and the ECU 24. For example, as for the operating condition of the air pump 60, the rotation number Nap [rpm] of the air pump 60 is read. As for the operating condition of the water pump 80, the rotation number Nwp [rpm] of the water pump 80 is read. As for the operating condition of the air conditioner 90, output settings of the air conditioner 90 are read.

In step S15, the ECU 24 calculates the electric power Pa [W] consumed by the auxiliary devices depending on the present operating conditions of the auxiliary devices.

In step S16, the ECU 24 calculates the sum of the estimated electric power Pm consumed by the motor 14 and electric power Pa consumed by the auxiliary devices (provisional system load Pm+Pa) to determine the estimated electric power consumption in the entire FC vehicle 10. That is, the ECU 24 calculates the system load Psys (Psys=Pm+Pa, also denoted as Psys.rarw.Pm+Pa).

As described above, the energy management according to the present embodiment is aimed to suppress degradation of the FC stack 40, and improve the efficiency in the output of the entire FC system 12.

FIG. 8 shows an example of the relationship between the voltage of the FC cell of the FC stack 40 (cell voltage Vcell) [V] and the degradation D of the cell. That is, a curve (characteristic) 142 in FIG. 8 shows the relationship between the cell voltage Vcell and the degradation D.

In FIG. 8, in a region below the voltage v1 (e.g., 0.5V), reduction reaction of platinum (oxidized platinum) in the FC cell proceeds severely, and aggregation of platinum occurs excessively (hereinafter referred to as the "platinum aggregation-increasing region R1" or the "aggregation-increasing region R1"). In a region from the voltage v1 to the voltage v2 (e.g., 0.8 V), reduction reaction proceeds stably (hereinafter referred to as the "stable platinum reduction region R2", the "stable reduction region R2", or the "region R2 of a voltage range where catalyst reduction proceeds stably").

In a region from the voltage v2 to the voltage v3 (e.g. 0.9 V), oxidation-reduction reaction of platinum proceeds (hereinafter referred to as the "platinum oxidation reduction progress region R3" or the "oxidation reduction progress region R3"). In a region from the voltage v3 to the voltage v4 (e.g., 0.95V), oxidation reaction of platinum proceeds stably (hereinafter referred to as the stable platinum oxidation region R4'' or the "stable oxidation region R4", or the "region R2 of a voltage range where catalyst oxidation proceeds stably"). In a region from the voltage v4 to OCV (open circuit voltage), oxidation of carbon in the FC cell proceeds (hereinafter referred to as the "carbon oxidation progress region R5").

As described above, in FIG. 8, if the cell voltage Vcell is in the stable platinum reduction region R2 or the stable platinum oxidation region R4, degradation of the FC cell occurs to a smaller extent. In contrast, if the cell voltage Vcell is in the platinum aggregation increasing region R1, the platinum oxidation reduction progress region R3, or the carbon oxidation progress region R5, degradation of the FC cell occurs to a greater extent.

In FIG. 8, on the face of it, a curve (characteristic) 142 is uniquely determined. However, in practice, the curve (characteristic) 142 varies depending on variation of the cell voltage Vcell (varying speed Acell) [V/sec] per unit time.

FIG. 9 is a cyclic voltammetry diagram showing an example of the progress of oxidation and the progress of reduction in the cases of different varying speeds Acell. In FIG. 9, a solid curve 170 (characteristic) shows a case where the varying speed Acell is high, and a dotted curve 172 (characteristic) shows a case where the varying speed Acell is low. As can be seen from FIG. 9, since the degree of the progress in oxidation or reduction varies depending on the varying speed Acell, the voltages vl to v4 (FIG. 8) cannot necessarily be determined uniquely. Further, the voltages vl to v4 (FIG. 8) may change depending on the individual difference in the FC cell. Therefore, preferably, the voltages vl to v4 (Fig. 8) should be set such that errors are reflected in the theoretical values, the simulation values, or the measured values.

Further, in the current-voltage (I-V) characteristic of the FC cell, as in the case of normal fuel cells, as the cell voltage Vcell decreases, the cell current Icell [A] is increased (see a I-V characteristic 162 indicated by "normal" in FIG. 10, and hereinafter referred to as "normal I-V characteristic"). Additionally, the power generation voltage (FC voltage Vfc) of the FC stack 40 is obtained by multiplying the cell voltage Vcell by the serial connection number Nfc in the FC stack 40. The serial connection number Nfc indicates the number of FC cells connected in series in the FC stack 40. The serial connection number Nfc is also simply referred to as the "cell number".

The description continues in the full USPTO document.